1. Field of the Invention
The present invention relates to an external cavity control semiconductor laser which is used in an all technical field using an optical signal source, especially In a technical field of optical coherent communication and optical coherent measuring.
2. Prior Art
FIG. 3 shows a construction of a conventional external cavity control semiconductor laser. In this Figure, the external cavity control semiconductor laser consists of a diffraction grating 2, an antireflection film 1A, a semiconductor laser 1, an optical isolator 6 and an optical fiber 8. Semiconductor laser 1 is a Fabry-Perot type having a coating of an antireflection film 1A at one end thereof, and is driven by an LD driver 9. In FIG. 3, semiconductor laser 1 outputs a laser beam from the antireflection film 1A. The laser beam is transformed to parallel light by a convex lens 5A, then is inputted to diffraction grating 2. Diffraction grating 2 is an external reflector, and reflects a light having a specific selected wavelength. The reflected light feeds back to semiconductor laser 1.
According to this construction, an external resonator is formed by the one end of semiconductor laser 1, which is not coated with the antireflection film 1A, and diffraction grating 2. As a result, semiconductor laser 1 resonates in a signal mode, and the laser from semiconductor laser 1 is directed to the optical fiber 8 through the convex lens 5B and the optical isolator 6. In the external resonator, if the length of the resonator is longer, it can narrow the spectral line width of the laser.
Next, when diffraction grating 2 having an angle adjusting mechanism 10A is rotated, the selected wavelength changes. Therefore, it is possible to change the wavelength within a gain range (a hundred and tens of nm) of the semiconductor laser 1. And, when diffraction grating 2 having a parallel displacement mechanism 11A is moved parallel to the light axis of the resonator, the phase condition of the resonator changes. Therefore, it is possible to change the wavelength within a frequency range (several GHz) of the longitudinal mode interval. Therefore, by controlling the angle and parallel displacement of diffraction grating 2 by adjusting mechanism 10A and parallel displacement mechanism 11A at same time respectively, it is possible to change the wavelength with continually changing phase.
For example, it is reported in PROCEEDINGS OF THE 1992 IEICE SPRING CONFERENCE, C-266, 1992, by Institute of Electronics, Information and Communication Engineers, that if the external resonator is 120 mm length, the spectral line width of about 10 kHz, and a frequency of 65 GHz are obtained at the longitudinal mode interval of 1.25 GHz (0.01 nm). It is also reported In IEICE TRANS. COMMUN., VOL. E75-B, NO. 6, JUN., 1992, pp.521-523 that if the external resonator is 30 mm length, the spectral line width becomes below 50 kHz, and the phase frequency becomes 130 GHz.
However, for the construction shown in FIG. 3, when the external resonator is made longer, the spectral line width can be made narrow, the longitudinal mode interval also becomes narrow, and as a result, single mode resonance will be difficult. And, for changing the wavelength with consecutive phase, the latitude of frequency adjusting becomes narrow, then it Is difficult to continuously change the phase. And, if the external resonator is longer, the construction of the resonator becomes a large, so that it becomes sensitive to an environment changes. Therefore, it is difficult to stabilize the wavelength of the laser source.